Suspicion intensified yesterday that a suicide car bomber who killed three US soldiers training Pakistani troops along the Afghan border had inside information on their movements.

If confirmed that Wednesday's suicide attack was aimed at the Americans, it would indicate an increased sophistication in militant tactics, as well as potential infiltration of extremists in Pakistani security forces.

The attack on US forces occurred in Lower Dir, a north-west district believed to be a crossroads for al-Qaida and the Taliban.

The blast also killed three schoolgirls and a Pakistani paramilitary soldier. Two more US soldiers were among dozens wounded.

The soldiers belonged to a group of American troops training Pakistan's paramilitary Frontier Corps.